The present invention is directed to improvements for differential transimpedance amplifiers, and more particularly to a circuit and method for reducing the output impedance and the common mode gain in a differential transimpedance amplifier.
A differential transimpedance amplifier converts a signal current to a voltage. For example, the output of a double balanced mixer cell is a differential current (the difference between two currents.) A differential transimpedance amplifier may be used to convert this differential current to a differential output voltage.
Transimpedance accuracy may be improved by decreasing the output impedance. A transimpedance amplifier may use feedback resistors to set a desired transimpedance value, and any additional output impedance associated with the high impedance end of the feedback resistor degrades transimpedance accuracy.
Further, in many applications for transimpedance amplifiers a common mode input current is not part of the desired output signal and the presence of a common mode current, such as mixer switching noise, will also degrade transimpedance amplifier performance. A common mode input current impact on the differential output voltage can be avoided in a well designed conventional differential transimpedance amplifier, but the common mode input current will still show up as a common mode signal from either of the amplifier's outputs to ground.
It is desirable that the voltage from either amplifier output to ground be the same as the differential output voltage (that is, the voltage between the two outputs), except at one-half the magnitude. As will be appreciated, a common mode signal from an output to ground degrades the accuracy of the desired voltage from the output to ground.
A single stage, differential transimpedance amplifier typical of the prior art is depicted in FIG. 1. The feedback resistors are resistors R1 and R2. In circuit variations, resistors R5 and R6 may be connected together, instead of grounded as shown. Resistors R3, R7 and R8 may be current sinks or sources, instead of resistors as shown.
The transimpedance values for the amplifier of FIG. 1 may be determined by conventional analysis in which it may be assumed that, the impedances of resistors R7 and R8 are counted as part of the load resistance R.sub.L1 or R.sub.L2, the impedances of resistors R5 and R6 account for the impedances at the input including the input impedances of the transistor bases and the output impedances of any preceding stages, and that symmetry permits half circuit analysis; that is, R7=R8, R.sub.L1 =R.sub.L2, R1=R2, R5=R6, and Q1=Q2. It will also be noted that the amplifier transconductance g.sub.m and the effective input impedance (represented by resistor R5) have different values for differential input currents (I1=-I2) and common mode input currents (I1=I2). For differential inputs, the transconductance g.sub.m of the amplifier will be g.sub.m1, the transconductance of the transistor Q1 (recall that Q1=Q2). For common mode inputs, g.sub.m is: ##EQU1##
The loaded transimpedance is: ##EQU2## the unloaded output impedance (R.sub.L .fwdarw..infin.) is: ##EQU3## and the loaded input impedance (the impedance as R5.fwdarw..infin.) is: ##EQU4##
It is desirable to improve differential transimpedance amplifier performance by decreasing output impedance and improving the rejection of common mode output voltages.
Accordingly, it is an object of the present invention to provide a novel circuit and method for improving differential transimpedance amplifier performance.
It is another object of the present invention to provide a novel circuit and method for a differential transimpedance amplifier in which an effective input impedance is generated that has one value for differential inputs and another value for common mode inputs.
It is yet another object of the present invention to provide a novel circuit and method for a differential transimpedance amplifier in which an effective input impedance is negative for differential inputs and positive for common mode inputs.
It is still another object of the present invention to provide a novel circuit and method for a differential transimpedance amplifier in which an input circuit for the amplifier sets the effective input impedance (R5) equal to the negative of the feedback resistor (R1) impedance for differential inputs.
It is a further object of the present invention to provide a novel circuit and method for a differential transimpedance amplifier in which an input circuit for the amplifier sets the effective input impedance (R5) to a low, positive value near zero for common mode inputs.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.